1. Field of the Invention
The invention relates to drag-type rotary drill bits for use in drilling holes in subsurface formations and of the kind comprising a bit body having a leading face and a gauge region, a plurality of cutting elements mounted on the bit body, a plurality of fluid channels extending outwardly away from the bit axis across said leading face and towards the gauge region and a plurality of nozzles for supplying fluid to the channels.
2. Description of Related Art
The invention is particularly, but not exclusively, applicable to drill bits in which some or all of the cutting elements are preform (PDC) cutters each formed, at least in part, from polycrystalline diamond. One common form of cutter comprises a tablet, usually circular or part-circular, made up of a superhard table of polycrystalline diamond, providing the front cutting face of the element, bonded to a less hard substrate which is usually of cemented tungsten carbide.
The bit body may be machined from solid metal, usually steel, or may be molded using a powder metallurgy process in which tungsten carbide powder is infiltrated with metal alloy binder in a furnace so as to form a hard matrix.
In the normal prior art construction the gauge region of the drill bit is formed by a plurality of kickers which are spaced apart around the outer periphery of the bit body and are formed with bearing surfaces which, in use, bear against the wall of the borehole. The kickers generally form continuations of respective blades formed on the leading face of the bit and extending outwardly away from the axis of the bit towards the gauge region so as to define said fluid channels between the blades. The spaces between the kickers define junk slots with which the channels communicate. During drilling, drilling fluid pumped down the drill string to the nozzles in the bit body flows outwardly along the channels, into the junk slots at the ends of the channels, and passes upwardly through the junk slots into the annulus between the drill string and the wall of the borehole.
While PDC bits have been very successful in drilling relatively soft formations, they have been less successful in drilling harder formations, including soft formations which include harder occlusions or stringers. Although good rates of penetration are possible in harder formations, the cutters may suffer accelerated wear and the bit life may be too short to be commercially acceptable.
Studies have suggested that the rapid wear of PDC bits in harder formations may be due to chipping of the cutters as a result of impact loads caused by vibration of the drill bit. One of the most harmful types of vibration can be attributed to a phenomenon called "bit whirl", in which the drill bit begins to precess around the hole in the opposite direction to the direction of rotation of the drill bit. One result of bit whirl is that some cutters may temporarily move in the reverse direction relative to the formation and this can result in damage to the cutters.
It is believed that the stability of such a drill bit, and its ability to resist vibration, may be enhanced by increasing the area of the bearing surfaces on the gauge region which engage the wall of the borehole. In most prior art designs, however, the area of engagement could only be increased by increasing the length and/or width of the bearing surfaces of the kickers. It may be undesirable to increase the length of the bearing surfaces since this may lead to difficulties in steering the bit in steerable drilling systems. Similarly, increasing the circumferential width of the bearing surfaces necessarily reduces the width of the junk slots between the bearing surfaces, and this may lead to less than optimum hydraulic flow of drilling fluid along the channels and over the cutters, or it may lead to blockage of the junk slots and channels by debris.
British Patent Specification No. 2294070 describes and claims certain arrangements for reducing or overcoming some of the above disadvantages. The specification describes a drill bit of the kind first referred to wherein there is provided at the outer end of at least one of the channels, in the gauge region, an additional bearing surface which extends across the whole width of the channel. The bearing surface necessarily inhibits flow of drilling fluid from the channel across the gauge region of the drill bit. In order to allow escape of drilling fluid flowing outwardly along the channel, therefore, there is provided in the channel, adjacent the gauge region, an opening into an enclosed passage which passes internally through the bit body to an outlet. The present invention provides a development of the invention described in GB 2294070.